1. Field of the Invention
The invention relates to an electromagnetic deflection unit and more particularly to an electromagnetic deflection unit for deflecting the electron beam or beams in a display tube which is used for displaying pictures in a television receiver.
2. Description of the Prior Art
With the advent of display tubes of large maximum deflection angle in the field of television, it has become more and more difficult to design and manufacture electromagnetic deflection units which deflect the electron beam or beams of said tubes in the correct manner to provide a commercially acceptable television frame on the associated screen. An electromagnetic deflection unit normally comprises two pairs of deflection coil units. One pair of coil units is used to provide vertical deflection of the electron beam(s), while the other pair is used for providing horizontal deflection of the beam(s). Display tubes which require a beam deflection over a large angle, more particularly color television tubes, require the generation of magnetic fields having an accurately defined configuration, or field distribution, both by the horizontal and by the vertical deflection coil units in order to correctly deflect the electron beam or beams in the tube.
The coil units for the horizontal or line deflection are generally constructed so as to fit around the neck and funnel portions of the cathode-ray tube and have saddle-type windings. The individual coil units are automatically wound on a winding machine. The coil units for the vertical deflection may also be of the saddle type or they may be of the torroidal type. In the latter case they are wound torroidally in the axial direction around the soft-magnetic yoke ring of the deflection unit.
Coil units of the saddle type are usually wound in a two-part mould (see U.S. Pat. No. 3,086,562). The outer circumference of the aperture in which the wire is to be guided is not situated in one plane but follows an intricate path.
Wing-shaped parts are connected to the two mould halves so as to slide the wire in the aperture. The winding wire has a thermoplastic layer of bonding material so that, after winding, a self-supporting coil can be obtained by means of heat and pressure.
It is furthermore known from laid-open Japanese Patent Application No. 57-23451 to wind a coil on a coil former in which the wire is guided by the winding machine and is fixed at discrete points by supplying an adhesive.
In order to achieve a reasonable reproducibility of the saddle coils, the utmost care should be paid to, inter alia, the shape of the moulds, the constancy of the outside wire diameter, the thickness of the layer of a thermoplastic bonding material, the smoothness of the wire, the softening temperature of the bonding layer, the wire tension during winding, the temperature of the mould during winding, and the winding speed.
Tolerances in the above-mentioned parameters, and also the process of arbitrary winding and the uncontrolled sliding of the wire from the wings of the mould during winding, have so far militated against a good reproducibility. This applies particularly in the case of coils the length of which is of the size of their winding height and the active parts (the flanks) of which are curved. A good reproducibility is achieved if in all coils of a series of deflection units each turn having the same sequence number is located in exactly the same position.